Corrosion metal loss is one of many causes of structural damage. One method of rapidly surveying long lengths of structure components (such as pipes, tubes, cables, rods and plates) for flaws from a single test position includes the long-range guided-wave technique. The technique may be well suited for long-term (tens of years) of online, structural health monitoring applications with permanently installed guided wave probes. In order for this approach to work well, long-term stability of both the guided wave probe and the coupling means may be essential for maintaining consistent measurements.
Dry coupling often requires continuous pressure to be uniformly applied between the probe and the structural component. However, if the surface conditions are imperfect, uniform pressure may not provide sufficient uniform coupling. In addition, maintaining the same pressure becomes difficult in the presence of thermal cycling due to differential expansion of the pipe and fixtures sued to couple the guided wave into the pipe.
Various polymer systems, such as epoxy, polyurethane, or wax, have been used as couplants to bond the probe to the structural component. However, coupling is difficult to maintain due to various environmental conditions, such as thermal cycling or vibration. Field trials indicate that even seasonal temperature variations, in the range of 0° F. to 100° F., may cause debonding.
Shear wave coupling gel has also been used to couple the probes to structural components. However, maintaining a layer of a shear wave coupling gel may be difficult due to changes in the viscosity of the couplant that may occur over time. In addition, the effectiveness of shear wave coupling gel may also vary with operating temperature.
Once uncoupled, monitoring with the probe may be problematic. In addition, in many applications, the probe may be somewhat inaccessible making repairs difficult, such as when the probe is used to measure pipe buried in the ground or when the pipe is insulated. Further, costs associated with the labor and equipment related to replacing probes may make repairs difficult. Accordingly, a need remains to further develop systems for coupling guided wave probes to structural components for long term monitoring applications.